linux-kernel - Re: [PATCH 1/2] mm/zswap: don't return LRU

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Date: Fri, 26 Jan 2024 10:01:41 -0800
From: Nhat Pham <nphamcs@...il.com>
To: chengming.zhou@...ux.dev
Cc: hannes@...xchg.org, yosryahmed@...gle.com, akpm@...ux-foundation.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH 1/2] mm/zswap: don't return LRU_SKIP if we have dropped
 lru lock

On Fri, Jan 26, 2024 at 12:31 AM <chengming.zhou@...ux.dev> wrote:
>
> From: Chengming Zhou <zhouchengming@...edance.com>
>
> LRU_SKIP can only be returned if we don't ever dropped lru lock, or
> we need to return LRU_RETRY to restart from the head of lru list.

Ooops. You're right! I just double checked and only LRU_REMOVED_RETRY
and LRU_RETRY indicate we might have dropped the lock. My bad.

>
> Actually we may need to introduce another LRU_STOP to really terminate
> the ongoing shrinking scan process, when we encounter a warm page

Yup. This is what I was trying (and failing) to do. To be honest, this
needs to be even stronger: short-circuit ALL concurrent/ongoing zswap
shrinker scan processes that are touching this memcg (as they will
also shrink into warmer regions going forward). But that's a bit more
engineering to do. LRU_STOP, which stops this scan process, would be a
good place to start.

> already in the swap cache. The current list_lru implementation
> doesn't have this function to early break from __list_lru_walk_one.
>
> Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>

Reviewed-by: Nhat Pham <nphamcs@...il.com>

> ---
>  mm/zswap.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 00e90b9b5417..81cb3790e0dd 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -901,10 +901,8 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
>                  * into the warmer region. We should terminate shrinking (if we're in the dynamic
>                  * shrinker context).
>                  */
> -               if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
> -                       ret = LRU_SKIP;
> +               if (writeback_result == -EEXIST && encountered_page_in_swapcache)
>                         *encountered_page_in_swapcache = true;
> -               }
>
>                 goto put_unlock;
>         }
> --
> 2.40.1
>